The present invention relates to bicycle frames, and more particularly to bicycle frames having increased rigidity and strength using an improved wishbone construction for one or both of the upper and lower rear stays.
Bicycle frames generally comprise a truss portion and upper and lower rear stays. The truss portion is made up of a pedal crank housing with a seat mast extending upwardly therefrom; a head tube, used for mounting the handlebars and front wheel forks; a top tube extending between the top portion of the seat mast and the head tube; and a down tube connecting the pedal crank housing and head tube. The upper and lower rear stays form a triangle with the truss portion and the rear wheel mounting brackets (commonly known as rear drop outs), with the upper stay connecting the rear wheel brackets to the upper portion of the seat mast and the lower stays connecting the rear wheel brackets to the pedal crank housing.
The rigidity of the frame affects the ease with which the bicycle can be pedaled. If the frame is not rigid, some of the energy expended during pedaling is used to cause deflection of the frame, rather than propelling the bike forward. This problem is particularly noticeable when pedaling light-weight "mountain bike" frames over uneven terrain. Of course, there are limits on the desired rigidity, particularly when increased rigidity can only be achieved with increased frame weight.
A number of bicycle frames have recently been manufactured using a "wishbone" shaped upper stay. Whereas prior art stays consisted of two separate pieces of tubing extending between the seat mast and the rear wheel brackets, wishbone stays have a U-shaped section, with separate tubes straddling the rear wheel, and a stem tube where the two side tubes are tied together. The stem tube connects on its other end to the seat mast.
These wishbone stays have generally consisted of three separate members, the two separate side tubes forming the legs of the U-shaped member, and the stem tube. Different configurations for attaching the two side tubes to the stem tube have been devised. One configuration requires bending the tops of the two side tubes inwardly and welding them to the sides of the stem tube. Another configuration uses a stem tube with a large, flared bottom end into which the ends of the two side tubes are inserted. A third configuration uses a cast wishbone lug for tying the three pieces together. These designs each have weaknesses due to the three piece nature of the rear stays.
Currently, many child-sized bicycles use a one piece, U-shaped tubular member for one or both of the rear stays. This design has not been used on adult bicycles primarily because the weld between the tubular member and seat mast or pedal crank housing is not very strong, due to the fact that there can only be point contact between the oppositely curved pieces being welded.
Some patented frame designs have used a single piece of tubing in a U shape to form forks for a bicycle. For example, U.S. Pat. No. 614,905 to Sturges discloses a lower rear stay constructed, in part, of a U-shaped tube. However, the U-shaped tube is connected to the pedal crank housing by a two piece yoke, which in turn is apparently held together by being inserted into a tubular portion extending from the pedal crank housing.
U.S. Pat. No. 2,353,712 to Dewey discloses front forks of a bicycle made from an unusually shaped tube that is larger in diameter in its central section, where it is flattened to an elliptical cross section, than at its ends. The tube is also bent into a U-shape. A similar U-shaped member is used for the front and rear stays. The U-shaped members, in each instance, are connected to the frame by a piece of tubing inserted through a hole in the top of the flattened, curved portion of the U-shaped member, and held in place by a ring weld.
These patented prior art designs are each more complicated, and would appear to provide less strength and rigidity to the frame than currently used wishbone stays. While the wishbone stays in use today have resulted in some improvement, additional strength and rigidity without excess weight is still desirable.